Edit:
Re-Read your post. There is no real good way to go about finding the Unit number of your vehicle, except to go to the Highway patrol office Historian for the state your car came from and hope they have some information. I tried to get some information from the Utah Highway patrol and found that they had purged the records some time ago.
If you mean the information from the VIN, try here first :
http://www.specialservicemustang.net...oder/index.php
Then Martini's website.